The present disclosure relates to methods for storing holographic data. Further, the present disclosure relates to holographic data storage media and articles having enhanced data storage capabilities which are derived from these methods.
Holographic storage is the storage of data in the form of holograms, which are images of three dimensional interference patterns created by the intersection of two beams of light, in a photosensitive medium. The superposition of a signal beam, which contains digitally encoded data, and a reference beam forms an interference pattern within the volume of holographic data storage medium. The holographic data storage medium typically comprises a photochemically reactive species which upon irradiation of the storage medium with the interference pattern results in a chemical reaction that changes or modulates the refractive index of the medium as a function of the characteristics of the interference pattern in a given volume element. This modulation serves to record as the hologram both the intensity and phase information from the signal. The hologram can later be retrieved by exposing the storage medium to the reference beam alone, which interacts with the stored holographic data to generate a reconstructed signal beam proportional to the initial signal beam used to store the holographic image. Thus, in holographic data storage, data is stored throughout the volume of the medium via three dimensional interference patterns.
Each hologram may contain anywhere from one to 1×106 or more bits of data. One distinct advantage of holographic storage over surface-based storage formats, including CDs or DVDs, is that a large number of holograms may be stored in an overlapping manner in the same volume of the photosensitive medium using a multiplexing technique, such as by varying the signal and/or reference beam angle, wavelength, or medium position. However, a major impediment towards the realization of holographic storage as a viable technique has been the development of a reliable and economically feasible storage medium.
Early holographic storage media employed inorganic photo-refractive crystals, such as doped or un-doped lithium niobate (LiNbO3), in which incident light creates refractive index changes. These refractive index changes are due to the photo-induced creation and subsequent trapping of electrons leading to an induced internal electric field that ultimately modifies the refractive index through a linear electro-optic effect. However, LiNbO3 is expensive, exhibits relatively poor efficiency, fades over time, and requires thick crystals to observe any significant index changes.
Therefore, there is a need for improved holographic data storage methods and materials through which enhanced holographic data storage capacities can be achieved. Further, there is also a need for methods to enhance the stability of the stored holographic data, such that for example, the data is not erased by exposure to ambient light, modest temperature changes, or during read-out.